Two phenomena known to inhibit the potential habitability of planets -- tidal forces and vigorous stellar activity -- might instead help chances for life on certain planets orbiting low-mass stars, University of Washington astronomers have found.
In a paper published this month in the journal Astrobiology, UW doctoral student Rodrigo Luger and co-author Rory Barnes, research assistant professor, say the two forces could combine to transform uninhabitable "mini-Neptunes" -- big planets in outer orbits with solid cores and thick hydrogen atmospheres -- into closer-in, gas-free, potentially habitable worlds.
Most of the stars in our galaxy are low-mass stars, also called M dwarfs. Smaller and dimmer than the sun, with close-in habitable zones, they make good targets for finding and studying potentially habitable planets. Astronomers expect to find many Earthlike and "super-Earth" planets in the habitable zones of these stars in coming years, so it's important to know if they might indeed support life.
Super-Earths are planets greater in mass than our own yet smaller than gas giants such as Neptune and Uranus. The habitable zone is that swath of space around a star that might allow liquid water on an orbiting rocky planet's surface, perhaps giving life a chance.
"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," Luger said. "Two important ones are strong tidal effects and vigorous stellar activity."
A tidal force is a star's gravitational tug on an orbiting planet, and is stronger on the near side of the planet, facing the host star, than on the far side, since gravity weakens with distance. This pulling can stretch a world into an ellipsoidal or egglike shape as well as possibly causing it to migrate closer to its star.
"This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide," Luger said. "Luckily, on Earth it's really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces."
This stretching causes friction in a planet's interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.
Vigorous stellar activity also can destroy any chance for life on planets orbiting low-mass stars. M dwarfs are very bright when young and emit lots of high-energy X-rays and ultraviolet radiation that can heat a planet's upper atmosphere, spawning strong winds that can erode the atmosphere away entirely. In a recent paper, Luger and Barnes showed that a planet's entire surface water can be lost due to such stellar activity during the first few hundred million years following its formation.
"But things aren't necessarily as grim as they may sound," Luger said. Using computer models, the co-authors found that tidal forces and atmospheric escape can sometimes shape planets that start out as mini-Neptunes into gas-free, potentially habitable worlds.
How does this transformation happen?
Mini-Neptunes typically form far from their host star, with ice molecules joining with hydrogen and helium gases in great quantity to form icy/rocky cores surrounded by massive gaseous atmospheres.
"They are initially freezing cold, inhospitable worlds," Luger said. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration." This process can bring mini-Neptunes into their host star's habitable zone, where they are exposed to much higher levels of X-ray and ultraviolet radiation.
This can in turn lead to rapid loss of the atmospheric gases to space, sometimes leaving behind a hydrogen-free, rocky world smack dab in the habitable zone. The co-authors call such planets "habitable evaporated cores."
"Such a planet is likely to have abundant surface water, since its core is rich in water ice," Luger said. "Once in the habitable zone, this ice can melt and form oceans," perhaps leading to life.
Barnes and Luger note that many other conditions would have to be met for such planets to be habitable. One is the development of an atmosphere right for creating and recycling nutrients globally.
Another is simple timing. If hydrogen and helium loss is too slow while a planet is forming, a gaseous envelope would prevail and a rocky, terrestrial world may not form. If the world loses hydrogen too quickly, a runaway greenhouse state could result, with all water lost to space.
"The bottom line is that this process -- the transformation of a mini-Neptune into an Earthlike world -- could be a pathway to the formation of habitable worlds around M dwarf stars," Luger said.
Will they truly be habitable? That remains for future research to learn, Luger said.
"Either way, these evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years."
Luger is lead author of the paper, with Barnes and Victoria Meadows his UW co-authors. Other co-authors are E. Lopez and Jonathan Fortney of the University of California, Santa Cruz, and Brian Jackson of Boise State University.
The research was done through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group, and funded through the NASA Astrobiology Institute under Cooperative Agreement Number NNA13AA93A .
This release is based on an essay by Luger. View a poster for the research. For more information, contact Luger at 206-543-6276 or email@example.com; or Barnes at 206-543-8979 or firstname.lastname@example.org.
Peter Kelley | EurekAlert!
Seeing the quantum future... literally
16.01.2017 | University of Sydney
Airborne thermometer to measure Arctic temperatures
11.01.2017 | Moscow Institute of Physics and Technology
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).
Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...
Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.
The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...
UMD, NOAA collaboration demonstrates suitability of in-orbit datasets for weather satellite calibration
"Traffic and weather, together on the hour!" blasts your local radio station, while your smartphone knows the weather halfway across the world. A network of...
Fiber-reinforced plastics (FRP) are frequently used in the aeronautic and automobile industry. However, the repair of workpieces made of these composite materials is often less profitable than exchanging the part. In order to increase the lifetime of FRP parts and to make them more eco-efficient, the Laser Zentrum Hannover e.V. (LZH) and the Apodius GmbH want to combine a new measuring device for fiber layer orientation with an innovative laser-based repair process.
Defects in FRP pieces may be production or operation-related. Whether or not repair is cost-effective depends on the geometry of the defective area, the tools...
10.01.2017 | Event News
09.01.2017 | Event News
05.01.2017 | Event News
16.01.2017 | Trade Fair News
16.01.2017 | Automotive Engineering
16.01.2017 | Life Sciences